perf_event_open.2: Use sizeof consistently

Use ``sizeof`` consistently through all the examples in the following
way:

- Use the name of the variable instead of its type as argument for
  ``sizeof``.

	Rationale:
	https://www.kernel.org/doc/html/v5.8/process/coding-style.html#allocating-memory

Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/perf_event_open.2

@@ -3439,7 +3439,7 @@ main(int argc, char **argv)
     printf("Measuring instruction count for this printf\en");
 
     ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
-    read(fd, &count, sizeof(long long));
+    read(fd, &count, sizeof(count));
 
     printf("Used %lld instructions\en", count);